In recent years, it has become increasingly necessary to provide improved mufflers for the attenuation of the exhaust noise produced by internal combustion engines used in transportation vehicles, due to public outcry resulting in government regulations. New regulations are almost continually being proposed which require ever more stringent noise standards, particularly as applied to heavy truck engines. Accordingly, it is necessary that the mufflers now available for truck engines be improved prior to implementation of these newer regulations.
Attenuation of sound can be accomplished in a variety of ways, but the constraints on designing a muffler for a transportation vehicle are such that limited methods of attenuation can be used. Inasmuch as a muffler for a transportation vehicle must be compact, inexpensive, durable under attack of hot and corrosive exhaust gases, and impervious to vibration and exposure to the weather, specialized attenuation means are generally used.
The sound to be attenuated is also of considerable significance in the design of an exhaust muffler. It will be understood that the exhaust noise from an internal combustion engine is typically comprised of a plurality of components, each of a specific frequency, which must each be attenuated. In accordance with well known prior art principles, attenuation of a wave may be accomplished by causing the wave to undergo a sufficient number of phase changes as to destroy the coherence of the wave; in the exhaust case, this turns the individual wave components of high amplitude into a multiplicity of waves of lesser amplitude, thus lowering the overall noise level. Accordingly, it is known in the art that mufflers to be efficient must comprise numerous means for alteration of the phase relationships of the individual waves making up the overall exhaust note.
Numerous prior art expedients are known for altering the phase relationship of sound waves. For example, it is known that to force incoming exhaust gases to make sharply angled turns alters the phase relationship, as does passing the gas flow through tubes of decreasing or increasing cross-sectional area. Other expedients include reflecting the gases from a terminus, thus causing them to interact with one another to create cancellation by means of destructive interference. Wave energy may also be absorbed by certain compressible materials, thus attenuating the wave energy. The exhaust flow may also be divided between two flow paths of differing length, thus causing destructive interference upon recombination of the diverted waves. Another possibiity is to incorporate a step in the exhaust passage, which also causes attenuation.
Numerous prior art disclosures show various combinations of these elements; see, for example, U.S. Pat. Nos. 1,881,051 and 2,235,705 to Haas which rely upon convoluted and plural flow paths to achieve attenuation. See also, U.S. Pat. No. 3,679,024 to Kirkland, Jr. et al., which shows a muffler providing an "S"-shaped flow pattern to the gases as well as using a venturi to throttle the gas flow.
A particularly relevant prior art patent is U.S. Pat. No. 3,672,464 to Rowley et al. which shows the use of a gradually divergent venturi following a sharply-contoured bell reducer used in conjunction with means for dividing the flow between paths of differing lengths to provide cancellation of the waves upon recombination.
A further consideration which is important, particularly in the heavy transportation context, is the provision of a muffler which provides efficient attenuation while not causing undue back pressure to the engine; that is, one which does not provide good muffling at the expense of engine operation efficiency.